Ice Ages Blamed on Tilted Earth

by Michael Schirber | March 30, 2005 04:09am ET

Ice Ages Blamed on Tilted EarthCredit:

In the past million years,
the Earth experienced a major ice age about every 100,000 years. Scientists
have several theories to explain this glacial cycle, but new research suggests
the primary driving force is all in how the planet leans.

The Earth's rotation axis
is not perpendicular to the plane in which it orbits the Sun. It's offset by
23.5 degrees. This tilt, or obliquity, explains why we have seasons and why
places above the Arctic Circle have 24-hour darkness in winter and constant sunlight
in the summer.

But the angle is not
constant - it is currently decreasing from a maximum of 24 degrees towards a
minimum of 22.5 degrees. This variation goes in a 40,000-year cycle.

Earth's Wobble ...

... is like the precession of a spinning top.

IMAGE: NASA

Peter Huybers of Woods Hole
Oceanographic Institution and Carl Wunsch of the Massachusetts Institute of
Technology have compared the timing of the tilt variations with that of the
last seven ice ages. They found that the ends of those periods - called glacial
terminations - corresponded to times of greatest tilt.

"The apparent reason
for this is that the annual average sunlight in the higher latitudes is greater
when the tilt is at maximum," Huybers told LiveScience in a
telephone interview.

More sunlight seasonally
hitting polar regions would help to melt the ice sheets. This tilt effect seems
to explain why ice ages came more quickly - every 40,000 years, just like the
tilt variations -- between two and one million years ago.

"Obliquity clearly was
important at one point," Huybers said.

Colder planet

The researchers speculate
that the glacier period has become longer in the last million years because the
Earth has gotten slightly colder - the upshot being that every once in a while
the planet misses a chance to thaw out.

The glacial cycles can be
measured indirectly in the ratio of heavy to light oxygen in ocean sediments.
Simply put, the more ice there is on Earth, the less light oxygen there is in
the ocean. The oxygen ratio is recorded in the fossils of small organisms -
called foraminifera, or forams for short - that make shells out of the
available oxygen in the ocean.

"These 'bugs' have
been around for a long time - living all across the ocean," Huybers said.
"When they die, they fall to the seafloor and become part of the sediment."

Drilled out sediment cores
from the seafloor show variations with depth in the ratio of heavy to light
oxygen - an indication of changes in the amount of ice over time. This record
of climate change goes back tens of millions of years.

By improving the dating of
these sediments, Huybers and Wunsch have showed that rapid decreases in the
oxygen ratio - corresponding to an abrupt melting of ice - occurred when the
Earth had its largest tilt.

Other orbital oddities

The significance of this
relationship calls into question other explanations for the frequency of ice
ages.

One popular theory has been
that the noncircular shape, or eccentricity, of Earth's orbit around the Sun
could be driving the glacial cycle, since the variations in the eccentricity have
a 100,000-year period. Curiously different, but interesting.

Variation in Orbit

Period

Tilt

40,000 yr

Wobble

20,000 yr

Eccentricity

100,000 yr

By itself, though, the
eccentricity is too small of an effect. According to Huybers, changes in the orbit
shape cause less than a tenth of a percent difference in the amount of sunlight
striking the planet.

But some scientists believe
a larger effect could be generated if the eccentricity fluctuations are coupled
with the precession, or wobble of the Earth's axis. It's like what is seen with
a spinning top as it slows down.

Earth's axis is currently
pointing at the North Star, Polaris, but it is always rotating around in a
conical pattern. In about 10,000 years, it will point toward the star Vega,
which will mean that winter in the Northern Hemisphere will begin in June
instead of January. After 20,000 years, the axis will again point at Polaris.

Huybers said that the
seasonal shift from the precession added to the eccentricity fluctuations could
have an important effect on glacier melting, but he and Wunsch found that the
combined model could not match the timing in the sediment data.

Skipping beats

The question, then, that
Huybers and Wunsch had to answer: How does the 40,000-year tilt cycle make a
100,000-year glacial cycle? A more careful sediment dating has shown is that
the time between ice ages may on average be 100,000 years, but the
durations are sometimes 80,000 years, sometimes 120,000 years -- both numbers
are divisible by 40,000. It appears there was not a mass melting every time the
tilt reached its maximum.

Did You Know?

101 Amazing
Earth Facts

"The Earth is skipping
obliquity beats," Huybers explained.

The planet only recently
started missing melting opportunities. Although the researchers have no
corroborating evidence, they hypothesize that the skipping is due to an overall
cooling of the planet.

The last major glacial thaw
was 10,000 years ago, which means that the Earth is scheduled to head into
another ice age. Whether human influences could reverse this, Huybers was
hesitant to speculate. Other researchers have found evidence that the process
of climate warming can set up conditions
that create a global chill.

"What we have here is
a great laboratory for seeing how climate changes naturally," he said.
"But this is a 100,000-year cycle, whereas global warming is happening a
thousand times faster."

Michael Schirber

Michael Schirber began writing for LiveScience in 2004 when both he and the site were just getting started. He's covered a wide range of topics for LiveScience from the origin of life to the physics of Nascar driving, and he authored a long series of articles about environmental technology. Over the years, he has also written for Science, Physics World, andNew Scientist. More details on his website.